Electrophotographic (EP) laser printing employs a toner containing pigment components and thermoplastic components for transferring a latent image formed on selected areas of the surface of an insulating, photoconducting material to an image receiver, such as plain paper, coated paper, transparent substrate (conducting or insulative), or an intermediate transfer medium.
There is a demand in the laser printer industry for multi-colored images. The image quality can be enhanced by a large number of approaches, including the technique which utilizes small particle developer including dry toner having an average particle size less than 5 .mu.m; see, e.g., U.S. Pat. Nos. 4,927,727; 4,968,578; 5,037,718; and 5,284,731. However, it has also been known that the electrophotographic dry toner having particle size less than 1 .mu.m is very hard to prepare due to increased specific area, and consequently, liquid toner has become one of the solutions for practical preparation of sub-micrometer xerographic developer.
Liquid toners comprise pigment components and thermoplastic components dispersed in a liquid carrier medium, usually special hydrocarbon liquids. With liquid toners, it has been discovered that the basic printing color (yellow, magenta, cyan, and black) may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate transfer medium to produce a multi-colored image.
The organic photoconductor products in the market today, generally speaking, are dual layer OPCs, which comprise a charge generation layer (CGL) and a charge transport layer (CTL) as key components. In addition to these layers, the photoconductor body can be undercoated or overcoated with other materials to improve adhesion to the substrate or to improve surface wear resistance or to reduce the surface adhesion for improved image transfer efficiency. The organic photoconductor (OPC) with an additional undercoating layer or overcoating layer becomes an organic photoreceptor (OPR) and ready for use in various designs of electrophotographic systems.
Most of the multilayer OPRs in the market are negative charging OPCs in which a thick hole transport layer is located on the top of a thin CGL. This is called the standard, or conventional, dual layer OPC. In the conventional case, the CGL usually comprises a photoconductive pigment or dye dispersed in an inert binder, with a pigment/dye content ranging up to about 90 wt %. 100% pigment in the CGL is possible where the pigment CGL is vacuum-evaporated in the format of a thin film; see, e.g., U.S. Pat. No. 4,578,334. Besides dispersion stabilizing functions, the CGL binder also plays an important role of adhesion.
Positive charging OPCs are also known, in which a thick electron transport layer is located on top of the thin CGL. Electron transport molecules are molecules which can transport an electron under a positive bias.
The advantages of the electron transport agent can be found in the design of a positive charging photoreceptor, in which the major carrier is the electron. In this design, the electron transport agent is also expected to provide excellent electrical stability of the photoreceptor, since it exhibits the least surface charge injection.
On the other hand, the challenges of the design of the electron transport molecules are associated with the solubility and the compatibility in various types of binders, inasmuch as electron transport agents, in general, are bulky.
A variety of electron transport agents have been disclosed, including derivatives of 4-thiopyran, dicyanofluorenone, imines, diphenobenzoquinone, and stilbene diphenobenzoquinone; see, e.g., U.S. Pat. Nos. 5,013,849; 5,034,293; and 5,213,923. However, 4-thiopyrans are expensive, most of the afore-mentioned compounds evidence poor compatibility with binders used to form the CTL, and most of these compounds suffer from a limited electron mobility range.
Thus, an electron transport agent is required which avoids most, if not all, of the problems associated with prior art electron transport agents.